Our research is directed toward understanding the genetic and cellular basis of immunoglobulin (Ig) regulation. Recently, we reported on a germline gene marker (U10-173) for Heavy (H) chain variable regions (VH) of mouse Igs (J. Exp. Med. 146: 1041, 1977). The near exclusive use of U10-173 in the anti-levan response of CWB mice (unpublished observation) will enable us to look for somatic variability of a germline gene product accompanying repeated immunization of CWB mice. Another aim of our research program is to identify what gene products and mechanisms enable certain T lymphocytes to recognize and inactivate allotype-producing B cells (J. Exp. Med. 145: 743, 1977). To investigate the mechanisms involved we are currentty studying allotype-specific interactions of T and B cells under in vitro conditions.